The invention relates to a device for monitoring wear on overhead lines for electrically-operated rail vehicles which include a current collector with a pair of contact bars.
The force with which a pair of contact bars of a current collector is pressed against the overhead line has an impact on both wear of the overhead line and wear of the contact bars. Measuring the contact force, and in particular measuring momentary changes in the contact force enables inference about damage to the overhead line or to the contact bars.
Various devices for measuring the contact force and for monitoring wear are known in the art. Document DE 102 49 896 B4 describes these correlations. Reference is made here to problems that may be caused when using force sensors with electrical strain gauges (SG), since the sensor system is subjected to a high voltage potential between 1.5 kV and 25 kV and is supplied via a battery for example. When using two motorcycle batteries, an operating time of about 24 hours can be realized. Power generation from the overhead line for this electrical force measurement technique is very complex and prone to fail. For that reason, document DE 102 49 896 B4 proposes to use sensors of the fiber Bragg grating type, designated FBG-sensors hereinafter, because they are not affected by the electrical high-voltage fields. However, manufacture of FBG-sensors with higher precision is difficult. On one hand, the thin optical fibers with the FBG-sensors can easily be attached to structural components which are at least partially subjected to the forces to be measured. However, this procedure leads to a poor measuring accuracy. Deformation bodies, used especially for conventional SG made of metal foil, can be equipped with FBG-sensors only to a limited extent, since the application of FBG-sensors requires significantly more space because of the high elasticity and the great bending radii of the optical fibers to be maintained during attachment. In other words, the attachment points on the deformation bodies for FBG-sensors have to be configured much larger, when compared to deformation bodies for electrical, i.e., metallic SG, so that the volume of the deformation body is increased overall.
In the intended field of application, there is the requirement that the used sensor system should not increase the air resistance at the current collector to more than 5%. The measurement systems known in the art are either relatively inaccurate or too bulky, i.e., the air resistance in travel direction is too high. The demand for a smallest possible air resistance in travel direction is not related to energy savings. The current collectors are constructed and calibrated such that even at high driving speeds they do not tend to vibrate. Mechanical vibrations can cause escalation of uncontrollable resonance effects and damage to the current collector or even to the overhead line. Since retrofitting of the sensor system also causes changes in the flow characteristic, i.e. the air resistance of the current collector, this limit value of maximal 5% is imposed.
A further, basic requirement for these force measurement systems involves lowest possible manufacturing costs and high mechanical sturdiness. When, for example, the overhead lines become iced, the deformation bodies or the sensors attached thereto should not be damaged. In order to meet this requirement, it is known in the art of force measurement and weighing technology to provide overload safeguards in the form of mechanical stops. Such overload safeguards, however, are bulky and thus additionally increase air resistance, if not aerodynamically integrated into existing construction elements of the current collector. Such an aerodynamic integration, however, always requires a special construction and therefore is expensive.
Consequently, the demands for high measurement accuracy, low manufacturing costs, and at the same time high mechanical sturdiness and slight air resistance are difficult to satisfy.